A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between two electrodes, an electric field is present in the dielectric layer. An ideal capacitor is characterized by a single constant value of capacitance which is a ratio of the electric charge on each electrode to the potential difference between them. In practice, the dielectric layer located between electrodes may pass a small amount of leakage current. Electrodes and leads introduce an equivalent series resistance, and dielectric layer has limitation to an electric field strength which results in a breakdown voltage.
A characteristic electric field known as the breakdown strength Ebd, is an electric field in which the dielectric layer in a capacitor becomes conductive. Voltage at which this occurs is called the breakdown voltage of the device, and is given by the product of dielectric strength and separation (distance) d between the electrodes,Vbd=Ebdd  (1)
The maximal volumetric energy density stored in the capacitor is limited by the value proportional to ˜∈·E2bd, where ∈ is dielectric permittivity of the dielectric layer and Ebd is breakdown dielectric strength. Thus, in order to increase the stored energy of the capacitor it is necessary to increase dielectric permeability ∈ and breakdown dielectric strength Ebd of the dielectric.
For high voltage applications, much larger capacitors can be used. There are a number of factors that can dramatically reduce the breakdown voltage. Geometry of the conductive electrodes is important for capacitor applications. In particular, sharp edges or points hugely increase the electric field strength locally and can lead to a local breakdown. Once a local breakdown starts at any point, the breakdown permeates through the dielectric layer and reaches the opposite electrode. Breakdown trace (electrically conductive channel) is conductive and causes a short circuit.
Breakdown of the dielectric layer usually occurs because intensity of an electric field becomes high enough to free electrons from atoms of the dielectric material and make them conduct an electric current from one electrode to another. Presence of impurities in the dielectric or imperfections (electric defects) of the crystal structure can result in an avalanche breakdown as observed in semiconductor devices. Thus, electrically conductive channels are formed in the dielectric layer when breakdown occurs. These channels have exit points on both surfaces of the dielectric layer.
Capacitors as energy storage device have well-known advantages versus electrochemical energy storage, e.g. a battery. Compared to batteries, capacitors are able to store energy with very high power density, i.e. charge/recharge rates, have long shelf life with little degradation, and can be charged and discharged (cycled) hundreds of thousands or millions of times. However, capacitors often do not store energy in small volume or weight as in case of a battery, or at low energy storage cost, which makes capacitors impractical for some applications, for example electric vehicles.